Bumping can be good or it can be bad. Airline bumping is what happens when a passenger is holding a confirmed ticket for a flight and the airline doesn't let you board because it overbooked the plane. You have to have purchased a ticket and checked in for the flight, either at the gate or up at the check-in desks. But if the airline bumps you, it provides travel on a future flight to the same city, and some type of compensation. The compensation is usually a voucher for future travel or a cash refund.
Bumping can happen voluntarily or involuntarily. In voluntary bumping, a passenger may see that the flight is full or overbooked and ask to be bumped or to have his or her name put on the bumping list. If a passenger is voluntarily bumped, the airline will typically be offering a voucher for a predefined amount, such as $300. Of course, the passenger would also receive a seat on the next flight to their destination. Many years ago, the vouchers were generally for a full one-way flight, but more recently most airlines provide a monetary voucher which may be less than a full one-way flight, depending on the route.
But bumping also happens involuntarily. That's when the airline denies you boarding, even if you have a confirmed seat. This also only happens in oversold situations, but it happens when no passenger volunteers to give up their seat. In the case that this happens to you, it's smart to demand cash instead of a flight credit because vouchers often come with a catch, such as blackout dates, economy seat selection only, etc. For more information on specific practices, ask the airline you're flying on for their rules and compensation policies for bumping.
How to Get Bumped
One of the most important tips for getting bumped is getting to the airport early. Check in for your flight, then ask the gate agent if your name can be put on a list for bumping, if the flight is in an oversold or full capacity. The second tip is to check back occasionally with the gate agent as it gets closer to departure time. Of course, you are more likely to get bumped from popular routes or travel times (like weekends, holidays, or peak seasons) that have a high number of passengers, and a high number of business travelers.
If you're hoping to snag some cash or credit, make sure to leave plenty of time in your schedule and plan to be flexible. Once you give up your seat, the next flight might not be for a few hours (meaning you could miss an important layover connection) or even the next day. If this is the case, the airline should cover your overnight stay at a hotel near the airport. Make sure to ask the gate agent these questions before volunteering. You don't want to be caught at a disadvantage because of the fine print.